I'm actually in the middle of a fanfic about that and i'm having teenage Digory and Polly go back during that time. But no I don't remember it being explained in LWW or in any other book. I think it was because CS Lewis wrote LWW first before writing MN and died before he could revise the books.

I'll give you a link to my fanfic, though it's not much I'm planning to start back on it soon.

http://www.fanfiction.net/s/5356276/1/Return_of_The_Magicians_Nephew

The protecting tree eventually died after hundreds of years. All trees die at a time, by disease, maybe hacked down by dwarfs or Narnians, because they eventually forgot the importance of the tree (and it was not a Dryad).
The White Witch could have sent one of her minions she gathered in the north to destroy the tree, after wich she released eternal winter.
Many possibilities, C.S. Lewis let's the answer to our imagination.

^ I agree with that. I haad always thought that the tree must have eventally fallen down. I wonder how they are going to work that out in the moive.....

Like DamselJill said. LLW was written first before CS Lewis even knew he was going to work on MN and never explained how. The movie never went into detail how she did it either, which really shows how much our movie crew knows

It's all up to our imaginations.

There are bound to be some background ideas Lewis had, but as he didn't share them in published books, we don't know officially.
But a fanfiction is NOT an answer, merely an idea.

Rilian's comments above are very good!